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The Extremely Large Telescope (ELT) hosts two Prefocal Stations, one on each of its Nasmyth Platforms A and B. Both units will alternately distribute the light collected by the telescope's giant optical system into the science instruments and other test equipment, deliver the focal surface images and pupil images of three natural guide stars for acquisition, guiding and wavefront sensing purposes and provide optical sensing to support phasing of the ELT primary mirrors, diagnostics, and maintenance of the optics as part of the Phasing and Diagnostics Station (supplied by ESO). As the subsystems were being assembled, the integration and verification activities proceeded gradually, following the typical V-process model approach. The full PFS Main System test without the M6 optics, which are being tested on their mirror mounts separately at the polisher premises, is now coming to its end and the overall system is ready for final testing.
Larringan et al. (Wed,) studied this question.